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Date:   Wed, 22 Sep 2021 08:32:08 +0200
From:   Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To:     Frederic Weisbecker <frederic@kernel.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Valentin Schneider <valentin.schneider@arm.com>,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        rcu@vger.kernel.org, linux-rt-users@vger.kernel.org,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>, Ingo Molnar <mingo@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        Steven Price <steven.price@arm.com>,
        Ard Biesheuvel <ardb@kernel.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Mike Galbraith <efault@gmx.de>
Subject: Re: rcu/tree: Protect rcu_rdp_is_offloaded() invocations on RT
Message-ID: <20210922063208.ltf7sdou4tr5yrnc@linutronix.de>
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On 2021-09-22 01:45:18 [+0200], Frederic Weisbecker wrote:
> 
> Also while at it, I'm asking again: traditionally softirqs could assume that
> manipulating a local state was safe against !irq_count() code fiddling with
> the same state on the same CPU.
> 
> Now with preemptible softirqs, that assumption can be broken anytime. RCU was
> fortunate enough to have a warning for that. But who knows how many issues like
> this are lurking?

If "local state" is modified then it is safe as long as it is modified
within a local_bh_disable() section. And we are in this section while
invoking a forced-threaded interrupt. The special part about RCU is
that it is used in_irq() as part of core-code.

> Thanks.

Sebastian